After the development of gallium nitride (GaN) based light emitting diodes, GaN-based LEDs have been applied to various fields such as natural color LED display devices, LED signboards, white LEDs, and the like.
In general, a gallium nitride based light emitting diode is formed by growing epitaxial layers on a substrate such as a sapphire substrate, and include an N-type semiconductor layer, a P-type semiconductor layer, and an active layer interposed therebetween. Then, an N-electrode pad is formed on the N-type semiconductor layer and a P-electrode pad is formed on the P-type semiconductor layer. For operation, the light emitting diode is electrically connected to an external power source through the electrode pads. At this time, current flows from the P-electrode pad to the N-electrode pad through the semiconductor layers.
On the other hand, in order to improve heat dissipation while preventing light loss by the P-electrode pad, a light emitting diode having a flip chip structure is used in the art, and various electrode structures have been suggested to help current spreading in a large area flip chip type light emitting diode (see U.S. Pat. No. 6,486,499). For example, a reflective electrode is formed on the P-type semiconductor layer, and extensions for current spreading are formed on a region of the N-type semiconductor layer exposed by etching the P-type semiconductor layer and the active layer.
The reflective electrode formed on the P-type semiconductor layer reflects light generated in the active layer to enhance light extraction efficiency and assists in current spreading in the P-type semiconductor layer. On the other hand, the extensions connected to the N-type semiconductor layer assist in current spreading in the N-type semiconductor layer such that light can be uniformly generated in a wide active area. Particularly, a light emitting diode having a large area of about 1 mm2 or more requires current spreading not only in the P-type semiconductor layer but also in the N-type semiconductor layer.
However, conventional techniques employ linear extensions causing limitation in current spreading due to high resistance thereof. Moreover, since a reflective electrode is disposed only on the P-type semiconductor layer, significant light loss occurs due to the pads and the extensions instead of being reflected by the reflective electrode.
Further, in the flip chip structure, light is emitted through a substrate. Accordingly, after semiconductor layers are formed on the substrate, a metallic reflective layer is formed above the semiconductor layers or a current spreading layer such that light can be reflected by the reflective layer.
FIG. 1 is a partial sectional view of a light emitting diode including a reflective layer in the related art.
Referring to FIG. 1, an ohmic layer 12 and a reflective layer 13 are disposed on a mesa layer 11. In addition, a barrier layer 14 surrounds a side surface of the ohmic layer 12 while surrounding an upper portion and side surface of the reflective layer 13.
The mesa layer 10 is a semiconductor area grown by epitaxial growth, and the ohmic layer 12 is composed of a conductive metal or a conductive oxide. In addition, the reflective layer 13 reflects light generated in the mesa layer 10 or a stack below the mesa layer. The reflective layer 13 is formed of sliver (Ag) or aluminum (Al).
The barrier layer 14 surrounding the upper portion and side surface of the reflective layer 13 has a structure wherein first barrier layers 14A and second barrier layers 14B are alternately stacked one above another. The first barrier layers 14A include nickel and the second barrier layers 14B include tungsten (W) or tungsten titanium (TiW). The barrier layer 14 prevents diffusion of metal elements constituting the reflective layer 13. On the other hand, the reflective layer 13 has a higher coefficient of thermal expansion than the barrier layer 14. For example, Ag has a coefficient of thermal expansion at room temperature of 18.9 um·m−1·K−1, and W has a coefficient of thermal expansion at room temperature of 4.5 um·m−1·K−1. Namely, there is a significant difference in coefficient of thermal expansion between the reflective layer 13 and the barrier layer 14.
Such a significant difference in coefficient of thermal between the reflective layer 13 and the barrier layer 14 induces stress in the reflective layer 13. Accordingly, the reflective layer 13 is separated from the ohmic layer 12 or the mesa layer 10 under the ohmic layer 12 due to stress generated in the reflective layer 13 at the same temperature.
On the other hand, various techniques have been developed to improve performance of the light emitting diode, that is, internal quantum efficiency and external quantum efficiency. Among various attempts to improve external quantum efficiency, a technique for improving light extraction efficiency has been developed in the art.